Paraffin-embedded archival tissue from 29 cases of malignant non-Hodgkin's lymphoma of T-cell type and 9 control lymph nodes were examined by flow cytometry for DNA aneuploidy and cell-cycle kinetics. DNA aneuploidy was detected in 4 cases (13%) and was not related to histologic grade. Proliferative activity, as measured by proliferative index and S-phase fraction, was significantly increased in lymphomas, compared with controls, and was significantly higher in morphologically high-grade lymphomas, compared with low-grade lymphomas and control lymph nodes (P less than 0.05). Different morphologic types of T-cell lymphoma were also significantly different in their proliferative activity P less than 0.05). Moreover, within the category of peripheral T-cell lymphoma, different proliferative rates occurred in tumors in which small cells predominated, compared with intermediate- and large-cell tumors, suggesting biologic differences within this group.